Ecdysozoa

Ecdysozoa is a protostome superphylum containing a vast number of animal phyla, characterized primarily by the process of ecdysis (molting). This shared characteristic is the shedding of an outer layer, usually a cuticle, during growth. Ecdysozoa is one of the two major clades within Protostomia, the other being Spiralia.

Key Characteristics:

• Ecdysis: The defining characteristic is the periodic shedding of an external cuticle to allow for growth. The cuticle is a non-living outer layer that provides protection and support. Molting is hormonally controlled.
• Protostome Development: As protostomes, Ecdysozoa exhibit spiral cleavage, and the blastopore typically develops into the mouth.
• Cuticle Composition: The cuticle is often composed of chitin, a complex polysaccharide.
• Phyla Included: Prominent phyla within Ecdysozoa include:
  • Arthropoda (insects, crustaceans, arachnids, myriapods)
  • Nematoda (roundworms)
  • Nematomorpha (horsehair worms)
  • Priapulida (penis worms)
  • Kinorhyncha (mud dragons)
  • Loricifera (brush heads)
  • Onychophora (velvet worms)
  • Tardigrada (water bears)

Phylogenetic Significance:

The Ecdysozoa clade was initially established based on molecular data, particularly ribosomal RNA sequences. Traditional morphology-based phylogenies often grouped Arthropoda with Annelida (segmented worms) due to their shared metamerism (segmentation). However, molecular evidence strongly supports the monophyly of Ecdysozoa, indicating that segmentation in Arthropoda and Annelida evolved independently. The grouping of these diverse phyla based on molecular data and the shared characteristic of ecdysis has revolutionized our understanding of animal evolution. The precise relationships within Ecdysozoa, particularly the basal relationships among the phyla, are still subject to ongoing research and debate.